Baking normal map for mutiple objects and merge them
Uncheck Clear option in the Bake panel. By doing this, you can bake the body and the wheels separately and with different options, but the normal map will be dinamically updated when you bake the second object. So your output will be a single normal map for the two objects, which stores normal data of both objects. Just make sure the two objects are using the same normal map as target bake texture.

This option is available both for Blender Internal and Cycles.

Blender Internal:



Cycles: